Recently, a liquid crystal display device or an organic EL display device has been often used as a flat panel display. By using an active matrix substrate in which a switching (active) element such as a thin film transistor (TFT) is formed so as to correspond to each display pixel, it is possible to enhance an ability of a display device. Such active matrix substrate is widely used for a PC (Personal Computer), a mobile phone, and the like.
In a case of forming a thin film transistor (TFT) on a glass substrate, an amorphous silicon film was used because heat-resistant temperature of the glass substrate is limited. Recently, by polycrystallizing the amorphous silicon film, it is possible to realize a polycrystalline silicon transistor with high performance in which mobility is greatly enhanced compared with an amorphous silicon transistor. By using a polycrystalline silicon film, it is possible to also provide a driving circuit on a single substrate. With such arrangement, the polycrystalline silicon film has been developed so as to realize higher performance and lower power consumption.
There is used a technology in which excimer laser light having a long-slit shape scans and polycrystallizes an amorphous silicon film. Pulse-oscillating excimer laser light is concentrated so as to have a long-slit beam shape, thus concentrated beam scans in a short-side direction of the beam, thereby efficiently polycrystallizing the amorphous silicon film with a large area.
Japanese Unexamined Patent Publication No. 229202/1998 (Tokukaihei 10-229202; published on Aug. 25, 1998) discloses that: when excimer laser light having a long-slit shape scans in a short-side direction, crystal grain sizes are even in a long-side direction, but there are formed a region with a large crystal grain size and a region with a small crystal grain size in a scanning direction, and therefore a channel length direction should be a direction in which mobility is not reduced by the region with a small crystal grain size (direction vertical to the scanning direction).
This disclosure depends on the fact that: whether the size of a crystal grain in a silicon film having been polycrystallized by excimer laser is large or not determines whether mobility is large or not. Further, it is disclosed that: in polycrystallization by excimer laser, crystalline characteristics differ according to a scanning direction.
Japanese Unexamined Patent Publication No. 243970/2000 (Tokukai 2000-243970; published on Sep. 8, 2000, corresponding U.S. Pat. No. 6,479,837 B1) discloses that: when a silicon thin film is scanned by KrF (XeCl) excimer laser light having a band shape (the laser light is even in a long side direction and has intensity distribution in a short side direction) and polycrystallized, a crystal grain of thus polycrystallized silicon thin film has an ellipse shape which is elongate in a scanning direction (e.g. grain size in a long side direction is 3 through 5 μm and grain size in a short side direction is 0.5 through 2 μm).
According to Japanese Unexamined Patent Publication No. 243970/2000, when a gate length direction and a long side direction of a crystal grain are substantially parallel to each other, high mobility (e.g. 480 cm2/Vsec) was obtained. Japanese Unexamined Patent Publication No. 243970/2000 suggests an arrangement in which a gate length direction of a thin film transistor is made substantially parallel to a direction in which higher characteristic is realized (hereinafter, this direction is referred to as a higher characteristic direction and means the scanning direction in which high mobility is realized) and accordingly mobility of a carrier is increased.
Further, Japanese Unexamined Patent Publication No. 86505/2003 (Tokukai 2003-86505; published on Mar. 20, 2003) discloses a technique in which an amorphous semiconductor film is patterned so as to have an insular shape and then solid laser (DPSS laser) with semiconductor (LD) excitation emits CW laser light to the amorphous semiconductor film from a back face of a transparent substrate, thereby polycrystallizing the amorphous semiconductor film. It is explained that: this method realizes a large crystal grain.
Polycrystallization using a continuous wave (CW) laser having a spot-like beam shape is performed in such a manner that: a semiconductor film is made insular and then scanned by the CW laser light so as to be polycrystallized. Polycrystalline silicon obtained through crystallization which is called “lateral growth” has a long crystal grain in a scanning direction. Mobility in a scanning direction is higher than mobility in a direction vertical to the scanning direction. A thin film transistor having a channel length direction parallel to the scanning direction has a higher characteristic than a thin film transistor having a channel length direction vertical to the scanning direction. Therefore, the scanning direction in which high mobility is realized is referred to as a higher characteristic direction and the direction which results in low mobility and is vertical to the scanning direction is referred to as a lower characteristic direction.
FIG. 7(A) schematically illustrates a TFT whose channel length direction is made parallel to a higher characteristic direction. A polycrystalline silicon film 101 has a higher characteristic direction D1 in a vertical direction and is elongate in a vertical direction and widens its width at both ends. Source/drain electrodes S/D are connected with the polycrystalline silicon film 101 at portions obtained by widening the polycrystalline silicon film 101. A gate electrode G is formed so as to cross an intermediate narrow portion of the polycrystalline silicon film 101 and a channel is defined in the polycrystalline silicon film 101 so as to be a portion where the gate electrode G overlaps with the polycrystalline silicon 101. A channel length direction D2 is a vertical direction and therefore coincides with the higher characteristic direction D1. At that time, high mobility can be obtained for the TFT.
FIG. 7(B) schematically illustrates a TFT whose channel length direction is vertical to the higher characteristic direction. A polycrystalline silicon film 102 has a higher characteristic direction D1 in a vertical direction and is elongate in a horizontal direction and widens its width at both ends. The source/drain electrodes S/D are connected with the polycrystalline silicon film 102 at portions obtained by widening the polycrystalline silicon film 102. A gate electrode G is formed so as to cross an intermediate narrow portion of the polycrystalline silicon film 102 and a channel is defined in the polycrystalline silicon film 102 so as to be a portion where the gate electrode G overlaps with the polycrystalline silicon 102. A channel length direction D2 is a horizontal direction and therefore crosses the higher characteristic direction D1. At that time, only low mobility can be obtained for the TFT.
An arrangement in which a driving circuit is integrated on an active matrix substrate of a liquid crystal display device is developed. The driving circuit of the liquid crystal display device includes a display controller and a shift register which should operate at high speed. It is preferable that a TFT which should operate at high speed has high mobility. It is also preferable that a transistor which needs a high driving ability, such as an analog switch, has high mobility.
FIGS. 7(C) and 7(D) schematically illustrate how to polycrystallize an active matrix substrate. As illustrated in FIG. 7(C), a display area 111 on which pixels are to be formed is defined at a central portion of a glass substrate 110. Pixel TFTs are disposed discretely so that each of the pixel TFTs corresponds to a pixel. Drain side driving circuit areas 112 are defined above and below the display area 111 and TFTs for a driving circuit are disposed in a high density manner. Gate side driving circuit areas 113 are defined at right and left of the display area 111 and the TFTs for a driving circuit are disposed in a high density manner. The drain side driving circuit areas 112 and the gate side driving circuit areas 113 are collectively termed “a peripheral circuit area”. Before polycrystallization is performed, an insular (ribbon-shaped) silicon film has been formed by patterning so as to correspond to each TFT.
As illustrated in FIG. 7(C), firstly, whole surfaces of the peripheral circuit areas on which insular silicon films are formed in a high density manner are scanned by CW laser light. CW laser light scans in a direction parallel to a longer side of each of the peripheral circuit areas, moves in a crossing direction, scans parallel to the longer side in an inverse direction, further moves in a crossing direction, and so on. With such scanning, there is formed a polycrystalline silicon film whose higher characteristic direction is parallel to a direction illustrated by a thick arrow in FIG. 7(C). In order to obtain high mobility, the insular silicon film is disposed so that the channel length direction is parallel to the higher characteristic direction.
As illustrated in FIG. 7(D), polycrystallization of the insular silicon film for a pixel TFT on the display area is subsequently performed. Pixels are disposed in a matrix manner on the display area and gate wiring is disposed so as to correspond to each pixel row and drain wiring is disposed so as to correspond to each pixel column. Wiring that extends from the gate wiring forms a gate electrode and wiring that extends from drain wiring forms a drain electrode. In such disposition, it is convenient that a direction combining a source and a drain is regarded as a row direction. The channel length direction is a horizontal direction and accordingly there is used an insular silicon film which is elongate in a horizontal direction. Polycrystallization is performed by irradiating CW laser light to insular silicon films with respect to each pixel row in a lateral direction. CW laser light is not irradiated in the column direction because the insular silicon films do not exist in the column direction.
The inventors of the present invention found that a problem occurs in the foregoing polycrystallization process.